1. Area of the Art
The present invention is in the field of treatment of drinking water to destroy water-borne pathogens and is more specifically in the area of treating water with iodine.
2. Description of the Prior Art
Water-borne disease has long been the bane of human civilization. As soon as human population density becomes significant contamination of drinking water becomes a problem. Many bacterial, viral and protozoan pathogens employ a fecal route of distribution. Generally, such pathogens can exist for a considerable—in some cases indefinite—period of time suspended in water. A small amount of infected human waste can contaminate a large volume of water. With steams and rivers the contaminating incident may occur out of sight upstream. With ground water human wastes can diffuse into wells and cause contamination from considerable distance. When an unsuspecting individual drinks such contaminated water without taking steps to destroy the pathogens, a disease state may rapidly ensue—and that person's wastes may spread further contamination.
Water and sewage treatment are major factors in making cities and other large human populations possible. Most of us take the safety of municipal drinking water for granted. However, one has only to spend time in the Third World to learn that there are many large and apparently modern cities where the tap water cannot be safely drunk. Contaminated water can be made safe to drink by any of a number of treatments that destroy pathogens in the water. The simplest treatment is undoubtedly boiling the water. Essentially all water-borne pathogens can be destroyed by a few minutes of exposure to boiling temperatures—at least at sea level. However, it is not practical to boil the water supply for an entire city let alone the water supply for a single home. Therefore, chemical treatments to destroy pathogens have become the leading methods for disinfection of water. Pathogens can be destroyed by reactive chemicals such as halogens. Most municipal water supplies are treated with chlorine, either as the free gas or as a reactive compound such as sodium hypochlorite and chlorine dioxide (or in some cases an organic compound such as Chloramine-T (n-chloro-para-toluene sulfonamide sodium salt)) may be used. Chlorine is a favored disinfecting agent because it has residual activity and continues to kill pathogens as the water flows through the pipes.
Optimal chlorination of water can be fairly complex especially where gaseous chlorine is employed. Consequently there has remained a need for a simpler disinfecting method to use for single wells or for portable use. For such purposes another halogen, iodine, has proven to be effective. While pure iodine is a solid that is only slightly soluble in water (0.3 g/l at 20° C.), solutions of iodine can be made in alcohol or potassium iodide, or water saturated with iodine can be employed. Effective concentrations of iodine can then be metered or injected into water using such a solution. Further, residual iodine is often more palatable than residual chlorine. A potential drawback of iodine is that unless extremely high concentrations of iodine are employed, it may take thirty minutes or more for the iodine to destroy all of the pathogens. Also, the iodine becomes converted into iodide so that iodine disinfected water generally has an appreciable iodide concentration. Iodide is used biologically in the synthesis of thyroid hormone, and the safety of long-term ingestion of water with increased iodide has not been established. Another problems is that iodine is sensitive to the temperature and pH of the water to be disinfected. At low temperatures (below about 10° C.) and/or low pH's iodine disinfection can be very slow.
Recently methods of using iodine bound to ion exchange resins have been developed for water treatment. Using iodine bound to resin simplifies the addition of iodine because a simple flow through cartridge can be provided that combines filters to remove particulates with the iodine dispensing resin. In theory all that is necessary is to control the flow rate of the water to achieve disinfection. It will be appreciated that there is an inverse relationship between flow rate and disinfection. If the flow rate is too high insufficient iodine is liberated from the resin to effect adequate destruction of pathogens. At a low flow rate disinfection may be excellent, but the volume of water produced is insufficient to meet requirements.
A potential advantage of resins is that ion exchange resins can also be used to remove iodide and residual iodine from the treated water. This gives the potential for easily purified water with no iodine or iodide traces. The drawback to this approach is that iodine normally needs to contact the pathogens for several minutes to destroy them. This suggests that a holding tank would be required between the iodine resin and the iodine removal resin. This is especially true because the levels of iodine released from iodinated resin are usually quite low. This greatly complicates the process and increases the size of the purification apparatus.
The present inventor has long been involved in using iodine to disinfect blood and other biological products. The requirements of a biological disinfection system are related to but somewhat different from the requirements for water treatment. In water treatment it is desired to completely kill the pathogens as quickly as possible in as large a volume of water (flow rate) as possible. In the case of water treatment with iodine it is desirable to have as little iodine or iodide as possible in the end product. Volume and speed, per se, are not as important in biological disinfection. It is important, however, to kill all pathogens with as little addition (iodine and iodide) to the final product as possible. Perhaps the most critical thing in biological disinfection is to avoid damaging the biological product undergoing disinfection.
Iodine is an effective disinfectant because it is chemically reactive and chemically damages the pathogens. However, in the case of trying to kill pathogens in blood plasma, the iodine is also capable of reacting with proteins and other biomolecules that comprise the blood plasma. As a result many iodine treatments that completely destroy the pathogens in blood plasma also severely damage critical blood proteins. The present inventor has worked hard to develop systems to kill pathogens while sparing labile proteins and other biomolecules. One approach has been to add iodine and then rapidly remove it (by binding or capture) before the labile proteins are damaged. One of the most successful approaches employed by the present inventor has been to use ion exchange resins to supply iodine and also to remove iodine and iodide. Reference is made to U.S. Pat. No. 6,045,787 to the present inventor. In that patent is disclosed the counterintuitive idea of blending iodine containing and iodine capturing resin in a single column. At the correct flow rate such a mixture does a superior job of disinfecting protein and other solutions without damaging labile biomolecules.
The mixed iodine/capture resins are also effective at disinfecting water. However, with blood plasma and similar biologic products, total treatment volumes are in the neighborhood of one to a few liters. Therefore, flow rates in the region of a few milliliters per minute are adequate. For water treatment flow rates of one to several liters per minute, or greater, are required. Even at high flow rates the mixed resin system of U.S. Pat. No. 6,045,787 is somewhat effective at killing bacteria. Unfortunately, at such high flow rates destruction of some types of virus is not adequate. This problem could be dealt with by constructing a very large column so that contact time of the pathogens with the resin would be significantly increased. This would, however, negate one of the attractive features of a resin based treatment system-compact size.